1. Field of the Invention
The present invention relates to a light amplification device using a bandpass filter and, more particularly, to a light amplification device comprising a control circuit for tuning the transmission center wavelength of the bandpass filter to the wavelength of an optical signal.
2. Description of the Prior Art
Among known direct light amplifiers for directly amplifying a light beam, there are the optical fiber amplifier which uses, as a light medium, a fiber which is obtained by doping a rare-earth element in its core, and a semiconductor light amplifier which utilizes dielectric emission phenomenon in the semiconductor. Each amplifier outputs an amplified optical signal and at the same time a spontaneous emission light beam generated in the amplification medium. When both the amplified optical signal and the spontaneous emission light beam are input to a light receiver, interference of these light components and interference of spontaneous emission light components occur, and these interferences become noise which degrades the signal-to-noise ratio of the light receiver. For this reason, a narrow-band bandpass filter which transmits only an optical signal is provided in the output section of an light amplifier. The transmission band of this narrow-band bandpass filter must be set at several fractions of a nm (nano-meter) to several nm (nanometer), so that the optical wavelength which can be used as an optical signal Is undesirably limited to the transmission band of this narrow-band bandpass filter. In order to eliminate this limitation, a tunable bandpass filter whose transmission band can be changed is used.
In use of the tunable bandpass filter, its transmission center wavelength must be tuned to the wavelength of an optical signal. For this purpose, a known light amplification device is used wherein the peak value of the intensity of the optical signal transmitted through the tunable bandpass filter, is detected, a transmission center wavelength at the time of the wavelength of the optical signal is determined, and the transmission center wavelength is tuned to the wavelength of the optical signal. The wavelength of the optical signal varies slightly with variations in the temperature of the transmission line or the like. There is proposed a light amplification device which dynamically tunes the transmission center wavelength of a tunable bandpass filter based on this wavelength variation.
FIG. 1 is a schematic diagram of a conventional light amplification device using a tunable bandpass filter. An optical signal input from an optical signal input terminal 11 is supplied to a first optical branching unit 12. One optical signal split by the first optical branching unit 12 is input to a light amplifier 13, and amplified. The amplified light is supplied to a bandpass filter 14, and the light transmitted therethrough is input to a second optical branching unit 15. One optical signal branched by the second optical branching unit 15 is supplied to an optical signal output terminal 16, and the other branched optical signal is supplied to a wavelength control circuit 17. The other optical signal split by the first optical branching unit 12 is input to the wavelength control circuit 17. A signal for controlling a transmission center wavelength is input from the wavelength control circuit 17 to the bandpass filter 14. The wavelength control circuit 17 compares the wavelengths of the optical signals input from the first optical branching unit 12 and the second optical branching unit 15 to control the transmission center wavelength of the bandpass filter 14 so as to make the wavelengths coincide with each other.
FIG. 2 is a schematic diagram of the wavelength control circuit 17 in FIG. 1. The optical signal input from an optical signal input terminal 21 is split by a third optical branching unit 23, and one split optical signal is input to a first photodetector 24. The other split optical signal is input to a first optical filter 25, and the input optical signal is attenuated therein with a transmission loss corresponding to its wavelength. The transmitted optical signal is the supplied to a second photodetector 26 in order to be converted into an electrical signal. The first and second photodetectors 24 and 26 are connected to a first differential amplifier 27. The difference between the output voltages of these photodetectors 24 and 26 is output from the first differential amplifier 27. The optical signal input from optical signal input terminal 22 is split by a fourth optical branching unit 31, and one split optical signal is supplied to a third photodetector 32 in order to be convened into an electrical signal. The other split optical signal is input to a second optical filter 33, and the optical signal transmitted therethrough is supplied to a fourth photodetector 34. The third and fourth photodetectors 32 and 34 are connected to a second differential amplifier 35. The first and second differential amplifiers 27 and 35 are connected to a third differential amplifier 36. The difference between the output voltages of the differential amplifiers 27 and 35 is output as a control signal from the third differential amplifier 36. This output is supplied to a control signal output terminal 37.
In this wavelength control circuit, the intensity of the optical signal received by the second photodetector 26 is lower than that received by the first photodetector 24 due to the transmission loss of the first optical filter. The transmission loss of the first optical filter 25 changes as a function of the wavelength of the optical signal to be transmitted. Therefore, the voltage output from the first differential amplifier 27 changes as a function of the wavelength of the optical signal. The voltage output from the second differential amplifier 35 changes as a function of the wavelength of a light transmitted through the second optical filter 33. For purposes of illustration assume that the first and second optical filters 25 and 33 have the same transmission loss characteristics. When the wavelengths of optical signals transmitted through the first and second optical filters 25 and 33 are the same, the voltages output from the first and second differential amplifiers 27 and 35 are the same. Therefore, the third differential amplifier 36 has no output voltage. In case of different wavelengths of optical signals, an output voltage appears at the third differential amplifier 36 because of the corresponding difference between the output voltages of differential amplifiers 27 and 35. The wavelength control circuit 17 controls the bandpass filter 14 so as not to generate an output voltage in the third differential amplifier 36. With this operation, the transmission center wavelength of the bandpass filter 14 can be tuned to the wavelength of the optical signal. This conventional light amplification device is disclosed in Japanese Unexamined Patent Publication No. 4-264532.
In this conventional light amplification device, wavelength differences cannot be detected with high precision unless the transmission characteristics of the two optical filters, the splitting ratios of two optical branching units, and the photo-electric sensitivities of the two photodetectors used in the wavelength control circuit are all respectively identical to each other. For this reason, these components must have a high precision. The characteristics of these components vary with changes in ambient temperature and deterioration over time. Therefore, it is not easy to stably tune the transmission center wavelength of the bandpass filter to the wavelength of an optical signal. In addition, a small wavelength difference cannot be detected unless the rate of change of the transmission loss corresponding to the wavelength of each optical filter is increased to a certain degree. If the rate of change of the transmission loss corresponding to the wavelength of the optical filter Is increased, the operating band of the optical filter is narrowed so as to limit the usable wavelength band of the optical signal. On the other hand, the optical signal's usable wavelength band can be widened In the light amplification device, which detects the peak value of the intensity of the optical signal transmitted through a bandpass filter, determines the transmission center wavelength simultaneously with the wavelength of the optical signal, and tunes the transmission center wavelength to the wavelength of the optical signal. However, the maximum intensity of a spontaneous emission light beam may be greater than the intensity of the optical signal, so the transmission center wavelength cannot be accurately tuned to the optical signal.